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          浅谈设计模式 - 迭代器模式（十一）
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        <h1 id="浅谈设计模式-迭代器模式（十一）"><a href="#浅谈设计模式-迭代器模式（十一）" class="headerlink" title="浅谈设计模式 - 迭代器模式（十一）"></a>浅谈设计模式 - 迭代器模式（十一）</h1><h1 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h1><p>迭代器模式通常只需要知道该模式的实现原理和了解结构图即可，在设计模式当中自己实现的情况几乎是没有的，所以这个模式简单的过一遍。</p>
<a id="more"></a>

<h1 id="什么是迭代器模式"><a href="#什么是迭代器模式" class="headerlink" title="什么是迭代器模式"></a>什么是迭代器模式</h1><p>定义：<strong>提供顺序的方法访问一个聚合对象的各个元素，同时又不会暴露内部的功能</strong></p>
<p>迭代器模式通过接管遍历的细节，让提供方不必关心迭代的具体细节，只需要提供对应的聚合对象即可。</p>
<blockquote>
<p> 迭代器模式和增强的FOR循环:</p>
<p> Jdk1.5之后，将泛型和增强for循环加入到语言体系，可以直接对于集合的内容进行for循环查看，其实本质上还是调用了iterator方法，而java通过语法糖的形式为我们进行的遍历的隐藏。</p>
</blockquote>
<h1 id="迭代器模式结构图"><a href="#迭代器模式结构图" class="headerlink" title="迭代器模式结构图"></a>迭代器模式结构图</h1><pre><code>迭代器模式的结构图如下：

![](https://gitee.com/lazyTimes/imageReposity/raw/master/img/20210310215705.png)</code></pre><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">Aggregate：聚合对象的共同接口，定义了具备遍历功能的聚合对象，通过定义创建迭代器的接口来建立与迭代器的组合</span><br><span class="line">ConcreateAggregate：具体的迭代器实现对象，通过接口方法返回具体的迭代器实现</span><br><span class="line">Iterator：迭代器接口，定义迭代器的统一规范，所以派生类都需要强制按照接口标准执行迭代器的实现。保证迭代器具备相似的行为。</span><br><span class="line">ConcreteIterator：具体的迭代器实现，内部聚合对象的同时，可以扩展迭代器的遍历方式，比如ListIterator。</span><br></pre></td></tr></table></figure>



<h1 id="迭代器模式特点"><a href="#迭代器模式特点" class="headerlink" title="迭代器模式特点"></a>迭代器模式特点</h1><ul>
<li>迭代器将遍历的细节进行了封装，<strong>聚合对象不需要在关心客户端如何操作内部的变量</strong>，而是通过委托给迭代器的形式交由迭代器去完成具体的遍历细节。</li>
<li>迭代器可以在<strong>不暴露内部结构的同时让外界可以访问到内部的聚合对象</strong>，所以即使是完全不同的对象也可以统一对待和处理。</li>
<li>迭代器是一种职责的转移，将遍历的工作从原本的聚合对象中进行独立，能在<strong>不改动数据结构的同时改变数据的操作方式</strong>。</li>
</ul>
<h1 id="迭代器的注意事项："><a href="#迭代器的注意事项：" class="headerlink" title="迭代器的注意事项："></a>迭代器的注意事项：</h1><ol>
<li>需要注意迭代器存在<strong>内部的迭代器</strong>和<strong>外部的迭代器</strong>，内部的迭代器供对象本身使用不对外开放，外部的迭代器通过方法返回给调用方使用。</li>
<li>每个责任对应一个区域，超过区域意味多了一个责任，遍历和数据操作实际上是两个操作，应该区分对待。</li>
<li>注意迭代器是如何体现单一职责的原则，他剥离了遍历对象的功能，将其封装到一个迭代器内部进行使用。</li>
<li>好的迭代器应当具备“快速失败机制”，目的是为了防止操作内部元素的指针越界，同时及时通知客户端遍历异常。</li>
<li><strong>如果想让方法不支持某些功能，最好使用异常机制提醒客户端我不支持某些操作。</strong></li>
</ol>
<h1 id="案例"><a href="#案例" class="headerlink" title="案例"></a>案例</h1><pre><code>光有理论还是不够的，这里配合代码讲解迭代器是如何实现解耦聚合对象的遍历的。</code></pre><h2 id="模拟场景："><a href="#模拟场景：" class="headerlink" title="模拟场景："></a>模拟场景：</h2><pre><code>由于迭代器在实际工作中使用概率 **几乎为0**，这里设置的场景比较简单

我们使用window最常见的文件系统来作为案例，我们通常进入不同的磁盘，首先就是对该磁盘下面的第一级目录进行遍历，同时我们根据遍历方式的不同，可以将页面展示为分组，按照时间排序，显示不同的详略信息.....这些功能的本质都是遍历，只不过遍历的形式不同，为了实现对于菜单的不同形式遍历，我们通过定义不同迭代器来完成这一个目标。

接着，我们会发现迭代的种类丰富还不够，我们还需要迭代其他的内容，比如任务管理器需要迭代不同的进程，同样也包含了排序的或者隐藏部分进程等等一系列的功能，所以需要让不同的对象可以支撑相似的迭代操作，并且可以自由的替换迭代的方式，当然这部分功能不会放入案例部分，案例部分为简单的迭代器实现。</code></pre><h2 id="具体实现"><a href="#具体实现" class="headerlink" title="具体实现"></a>具体实现</h2><pre><code>在进行具体的编码之前，先检查一下需要的基本构建类：</code></pre><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">Travelsable：定义对象具备迭代的功能接口。</span><br><span class="line">MissionBoard：任务栏，贮存基本的任务信息，提供迭代器供外部展示。</span><br><span class="line">TaskItemlIterator：任务迭代器，用于任务的迭代操作</span><br><span class="line">ConcreteCatalogIterator：目录迭代器的实现具体子类，定义了不同的迭代种类。</span><br><span class="line">TaskItem 任务项，定义一个任务的内容</span><br><span class="line">Computer 电脑，只需要管理任务栏和文件管理器即可。</span><br><span class="line">FileManager 文件管理器，负责管理文件夹的内容</span><br><span class="line">FileIterator 文件迭代器，管理文件夹的迭代操作</span><br><span class="line">FileItem 文件项</span><br></pre></td></tr></table></figure>



<pre><code>下面直接按照结构图构建具体代码：</code></pre><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br><span class="line">95</span><br><span class="line">96</span><br><span class="line">97</span><br><span class="line">98</span><br><span class="line">99</span><br><span class="line">100</span><br><span class="line">101</span><br><span class="line">102</span><br><span class="line">103</span><br><span class="line">104</span><br><span class="line">105</span><br><span class="line">106</span><br><span class="line">107</span><br><span class="line">108</span><br><span class="line">109</span><br><span class="line">110</span><br><span class="line">111</span><br><span class="line">112</span><br><span class="line">113</span><br><span class="line">114</span><br><span class="line">115</span><br><span class="line">116</span><br><span class="line">117</span><br><span class="line">118</span><br><span class="line">119</span><br><span class="line">120</span><br><span class="line">121</span><br><span class="line">122</span><br><span class="line">123</span><br><span class="line">124</span><br><span class="line">125</span><br><span class="line">126</span><br><span class="line">127</span><br><span class="line">128</span><br><span class="line">129</span><br><span class="line">130</span><br><span class="line">131</span><br><span class="line">132</span><br><span class="line">133</span><br><span class="line">134</span><br><span class="line">135</span><br><span class="line">136</span><br><span class="line">137</span><br><span class="line">138</span><br><span class="line">139</span><br><span class="line">140</span><br><span class="line">141</span><br><span class="line">142</span><br><span class="line">143</span><br><span class="line">144</span><br><span class="line">145</span><br><span class="line">146</span><br><span class="line">147</span><br><span class="line">148</span><br><span class="line">149</span><br><span class="line">150</span><br><span class="line">151</span><br><span class="line">152</span><br><span class="line">153</span><br><span class="line">154</span><br><span class="line">155</span><br><span class="line">156</span><br><span class="line">157</span><br><span class="line">158</span><br><span class="line">159</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 电脑，只需要管理任务栏和文件管理器即可。</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Computer</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> FileManager fileManager;</span><br><span class="line">    <span class="keyword">private</span> MissionBoard missionBoard;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">Computer</span><span class="params">(FileManager fileManager, MissionBoard missionBoard)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.fileManager = fileManager;</span><br><span class="line">        <span class="keyword">this</span>.missionBoard = missionBoard;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">display</span><span class="params">()</span></span>&#123;</span><br><span class="line">        Iterator fileManagerIterator = fileManager.createIterator();</span><br><span class="line">        Iterator missionBoardIterator = missionBoard.createIterator();</span><br><span class="line"></span><br><span class="line">        <span class="keyword">while</span> (fileManagerIterator.hasNext())&#123;</span><br><span class="line">            Object next = fileManagerIterator.next();</span><br><span class="line">            System.out.println(next);</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        <span class="keyword">while</span> (missionBoardIterator.hasNext())&#123;</span><br><span class="line">            Object next = missionBoardIterator.next();</span><br><span class="line">            System.out.println(next);</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 文件项</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">FileItem</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> String fileName;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> String editDate;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> String ceateDate;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">long</span> size;</span><br><span class="line"></span><br><span class="line">   	<span class="comment">// 省略部分内容</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 任务项</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">TaskItem</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> String name;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> size;</span><br><span class="line"></span><br><span class="line">    <span class="comment">// 省略部分内容</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 迭代器规范接口</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Travelsable</span>&lt;<span class="title">E</span> <span class="keyword">extends</span> <span class="title">Object</span>&gt;</span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 创建迭代器的方法</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@return</span></span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function">Iterator&lt;E&gt; <span class="title">createIterator</span><span class="params">()</span></span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 任务栏，管理任务项. 实现接口，支持迭代操作</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">MissionBoard</span> <span class="keyword">implements</span> <span class="title">Travelsable</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> Stack&lt;TaskItem&gt; taskItems;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">MissionBoard</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        taskItems = <span class="keyword">new</span> Stack&lt;&gt;();</span><br><span class="line">        taskItems.push(<span class="keyword">new</span> TaskItem(<span class="string">"任务1"</span>, <span class="number">10</span>));</span><br><span class="line">        taskItems.push(<span class="keyword">new</span> TaskItem(<span class="string">"任务2"</span>, <span class="number">1230</span>));</span><br><span class="line">        taskItems.push(<span class="keyword">new</span> TaskItem(<span class="string">"任务3"</span>, <span class="number">123</span>));</span><br><span class="line">        taskItems.push(<span class="keyword">new</span> TaskItem(<span class="string">"任务4"</span>, <span class="number">414</span>));</span><br><span class="line">        taskItems.push(<span class="keyword">new</span> TaskItem(<span class="string">"任务5"</span>, <span class="number">555</span>));</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Iterator <span class="title">createIterator</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">new</span> TaskItemlIterator(taskItems);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 文件管理器，管理文件项，实现接口并且支持迭代操作</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">FileManager</span> <span class="keyword">implements</span> <span class="title">Travelsable</span></span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> FileItem[] fileItems;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">FileManager</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.fileItems = <span class="keyword">new</span> FileItem[<span class="number">10</span>];</span><br><span class="line">        Random random = <span class="keyword">new</span> Random(<span class="number">10000</span>);</span><br><span class="line">        <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; fileItems.length; i++) &#123;</span><br><span class="line">            fileItems[i] = <span class="keyword">new</span> FileItem(<span class="string">"文件"</span>+i, random.nextInt(<span class="number">2000</span>));</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Iterator <span class="title">createIterator</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">new</span> FileIterator(fileItems);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">//文件迭代器</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">FileIterator</span> <span class="keyword">implements</span> <span class="title">Iterator</span></span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> FileItem[] fileItems;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> index;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">FileIterator</span><span class="params">(FileItem[] fileItems)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.fileItems = fileItems;</span><br><span class="line">        <span class="keyword">this</span>.index = <span class="number">0</span>;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">hasNext</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> index++ &lt; fileItems.length - <span class="number">1</span>;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Object <span class="title">next</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> fileItems[index];</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">remove</span><span class="params">(Object ele)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> UnsupportedOperationException(<span class="string">"数组不支持当前操作"</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 任务迭代器</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">TaskItemlIterator</span> <span class="keyword">implements</span> <span class="title">Iterator</span></span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> Stack&lt;TaskItem&gt; taskItems;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">TaskItemlIterator</span><span class="params">(Stack&lt;TaskItem&gt; taskItems)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.taskItems = taskItems;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">hasNext</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">if</span>(taskItems.isEmpty())&#123;</span><br><span class="line">            <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line">        &#125;</span><br><span class="line">        TaskItem peek = taskItems.peek();</span><br><span class="line">        <span class="keyword">return</span> peek != <span class="keyword">null</span>;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Object <span class="title">next</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> taskItems.pop();</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">remove</span><span class="params">(Object ele)</span> </span>&#123;</span><br><span class="line">        taskItems.remove(ele);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>以上就是迭代的大致实现案例代码，现代编程基本不会自己去设计迭代器，所以了解概念和知道样板代码即可。</p>
<h1 id="总结："><a href="#总结：" class="headerlink" title="总结："></a>总结：</h1><pre><code>迭代器在JAVA语言中基本已经实现到集合当中，当我们遍历集合的时候，其实就是在使用迭代器，迭代器通过**封装遍历**解耦了一堆对象的遍历和创建工作，将迭代的细节封装到一个黑盒当中，外部只需要调用接口就可以操作集合的数据。</code></pre><script type="text/javascript" src="https://cdn.jsdelivr.net/npm/kity@2.0.4/dist/kity.min.js"></script><script type="text/javascript" src="https://cdn.jsdelivr.net/npm/kityminder-core@1.4.50/dist/kityminder.core.min.js"></script><script defer="true" type="text/javascript" src="https://cdn.jsdelivr.net/npm/hexo-simple-mindmap@0.2.0/dist/mindmap.min.js"></script><link rel="stylesheet" type="text/css" href="https://cdn.jsdelivr.net/npm/hexo-simple-mindmap@0.2.0/dist/mindmap.min.css">
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